GIF files contain pictures. They are widely used on the internet because they do a better job than other formats like JPEG for things like logos, buttons, and other web page graphics.
GIF files can also be animated – the file will contain a series of pictures which your web browser will display in sequence.
If you are downloading files from Usenet, you may want to tell your software to ignore GIF files since they will usually be spam. Serious photographers use JPEG files for their photos.